Discharge lamp thermoionic cathode containing emission material

ABSTRACT

Thermionic electrodes for high intensity discharge lamps, particularly sodium vapor lamps, contain dibarium calcium tungstate, Ba2CaWO6, as emission material. At temperatures over 1000*C, the material has good electron emission coupled with an evaporation rate appreciably lower than that of other bariumcontaining emission materials. Lamps show negligible voltage rise, maintenance of 89 percent at 14,000 hours, and life in excess of 14,000 hours.

United States Patent [191 Smyser et al.

[541 DISCHARGE LAMP THERMOIONIC CATHODE CONTAINING EMISSION MATERIAL Inventors: William E. Smyser, Chagrin Falls; Dimitrios M. Speros, Painesville, both of Ohio [73] Assignee: General Electric Company [22] Filed: Dec. 14, 1970 [2]] Appl. No.: 97,907

US. Cl ..313/213, 313/184 Int. Cl. ..H01j 17/04 Field of Search....3l3/109, 184, 346 R, 346 DC,

[56] References Cited UNITED STATES PATENTS 3,294,998 12/1966 Niles "313/184 X 5221 REGION OF 1 Jan. 2, 1973 3,434,812 3/1969 Bondley ..313/346 R Primary Examiner-Nathan Kaufman Attorney-Ernest W. Legree, Henry P. Truesdell, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [5 7] ABSTRACT Thermionic electrodes for high intensity discharge lamps, particularly sodium vapor lamps, contain dibarium calcium tungstate, Ba CaWO as emission material. At temperatures over 1000C, the material has good electron emission coupled with an evaporation rate appreciably lower than that of other bariumcontaining emission materials. Lamps show negligible voltage rise, maintenance of 89 percent at 14,000 hours tan m i e ns s s f 9 9 10 8 Claims. ,5 Dr win F sw 5H T/SFRCTUEY EMISSION M/QTEE/F/LS.

POOP ELECTEGN EMISSION.

MEL TING, HIGH Ba Ca W0 PATENTEU 2 I973 3,708,710 sum 2 OF 3 CaO Fig 3.

[:3 EEG/0N 0F 5%) T/SFHCTOEY EMISSION MAYTEE/FILS.

POOP ELECTRON EMISSION.

MEL TING, HIGH b EVHPOPflT/ON FATE, P002 ELECTEON EMISSION.

Invervtors: WiLLiam E. Smgser Dimktrios M. Spew-os DISCHARGE LAMP THERMOIONIC CATHODE CONTAINING EMISSION MATERIAL BACKGROUND OF THE INVENTION The invention relates to thermionic cathodes for electric discharge lamps or devices having a gas or vapor filling. Such cathodes generally comprise a base or support member of refractory metal, commonly tungsten, supporting electron emissive material which is more active than the base metal. The invention is particularly concerned with such cathodes for use in high intensity discharge lamps of the kind utilizing selfheated electrodes, that is electrodes heated by the discharge and not by current from an external source.

High intensity sodium vapor lamps of the kind described in US. Pat. No. 3,248,590 Schmidt, entitled High Pressure Sodium Vapor Lamp, require such electrodes. These lamps utilize a slender tubular envelope of a light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina. The filling comprises sodium along with a rare gas such as xenen to facilitate starting and mercury for improved efficiency. The ends of the alumina tube are sealed by refractory metal closure members, suitably niobium end caps bonded by a glassy sealing material. Each end cap supports an electrode extending along the axis of the tube comprising a tungsten rod having a double coil of tungsten wire wound around its inner end. The electrode is coated with or contains a quantity of low work function electron emissive material usually in the form of metal oxides including barium oxide retained in the interstices of the coil as a reservoir.

The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The end caps of the arc tube are connected to the terminals of the screw base, that is shell and center contact. The interenvelope space is usually evacuated in order to conserve heat.

SUMMARY OF THE INVENTION The cathodes are required to provide copious electron emission and to be resistant to vaporization and ion bombardment. However, these properties do not in general go together. Thus, for example, an alkaline earth metal oxide such as barium oxide is more active as an electron emitter than say barium thorate but it is not so resistant to vaporization and ion bombardment. The object of the invention is to provide a cathode with electron emissive material which is a good emitter and at the same time more resistant to vaporization and ion bombardment when used in a high pressure sodium vapor lamp than the materials which have been available heretofore.

We have discovered that dibarium calcium tungstate Ba CaWO is a better electron-emitting material for use in high intensity discharge lamps and particularly high pressure sodium vapor lamps than any material up to now. The compound is a very effective electron emitter at temperatures above I000C which is probably attributable to the fact that it contains barium and calcium, both good emitters, and has two barium atoms per molecule, barium being one of the most active electron emitters. At the same time, Ba CaWO is quite stable and its evaporation rate is appreciably lower than that of other barium containing emission materials. The end result is that lamps utilizing this emission material have higher efficacy, better maintenance, and longer life.

DESCRIPTION OF DRAWINGS In the drawings:

FIG. 1 is a ternary phase equilibrium diagram of the system CaOBaO-WO at l200C.

FIG. 2 is a diagram of the same system at 1400C and illustrating fusion phenomena.

FIG. 3 is a compositional diagram of the material of our invention.

FIG. 4 illustrates a jacketed high pressure sodium vapor lamp wherein our inventionis embodied.

FIG. 5 is a sectioned view of one end of the arc tube to a larger scale showing the details of the electrode.

PREPARATION OF MATERIAL Dibarium calcium tungstate, Ba cawO for use as an emission material may be prepared as a single phase by a variety of techniques well known in the chemical or ceramic art. The simplest technique is to react in air the correct proportions of barium carbonate, calcium carbonate and either tungsten oxide (W0 WO or tungstic acid corresponding to a molar ratio of 2:1:1 at some temperature between 1000C and l500C until reaction is complete. An outline of suitable procedure for synthesis of small samples 50 gm) of the material is as follows:

1. Using an alumina mill and enough acetone or alcohol for a creamy consistency in the mill, ball mill for two hours BaCO CaCO and W0 (mol. wt. 23 l .38) in the molar proportions 2:1 :1

2. Dry the contents by nitrogen or air flow into the mill, separate the powder from the balls by screening through a nylon screen, and dry the powder for two hours at l 10C.

3. Place the powder in an alumina crucible and fire in air from room temperature to 1200C, hold 4 hours at l200C, and cool back to room temperature. The resultant material is soft and easily friable, i.e. very little sintering occurs. X-ray powder diffraction shows that the reaction is complete and only the compound Ba CawO is observed. The material has a slightly offwhite body color. Preparation of larger samples requires additional milling and firing steps until reaction, as monitored by X-ray diffraction, is complete. We have found it advisable to fire samples larger than 1 kgm. at 1300C for 4 hours, remill and then fire again at 1300C for 6 hours.

The finished Ba CaWO is milled in a suspending medium methanol is convenient and painted onto the electrode bare metal. Other techniques for applying the emission material such as vacuum impregnation, use of binders, etc. known to electrode technology, have been used successfully.

As an alternative method of preparation, the milled suspension following the first step above may be applied directly to the electrode. Reaction of the components takes place within the lamp during the normal sealing process which is carried out in a vacuum furnace at high temperature. Any gases resulting from the reaction are exhausted by the furnace exhaust system and the compound Ba CaWO is formed directly on the electrodes or within the interstices between the turns of the electrode coils. Formation of Ba CaWO outside the lamp as in the first procedure, is preferred since the material can then be characterized by a series of chemical and physical measurements to insure optimum performance.

Consideration of the phase relationships, rates of evaporation, and work functions for electron emission of related compositions makes possible a delineation of the preferred compositions in the scope of the invention. Referring to FIG. 1, it is seen that Ba- CaWO is the only ternary compound present in the system CaO- BaO-WO All other compounds on the edges of the diagram are binary. There are three regions of solid solutions in the ternary system at 1200C as indicated in FIG. I. The Ba CaWO phase is that desired but emission material which consists of a Ba CaWO solid solution phase or a solid solution phase together with small amounts of binary phases are also satisfactory.

FIG. 2 shows compositions in the system that are partially or fully melted at l400C. In accordance with our invention, we exclude as unsuitable all compositions that melt at electrode operating temperatures or at temperatures obtained during sealing of the arc tube.

On the basis of the foregoing plus consideration of measured evaporation rates and work functions, we have delineated in FIG. 3 the compositional area centered about the compound Ba CaWO wherein satisfactory emission material is encountered. Compositions to the right of the line abc and relatively high in W are not desirable since they melt at electrode operating temperatures as previously explained; their electron emission and material evaporation rates are also unacceptable. Compositions to the left of line def and relatively high in BaO have an evaporation rate many times higher than Ba CaWO any initial advantage of these compositions due to higher electron emission is rapidly dissipated as the BaO, which is present as a physical mixture outside the range of solid solubility, evaporates. Compositions above line gbh and having a mole fraction of CaO greater than 0.30 are not desirable due to insufficient electron emission. The cross-hatched four-sided figure about point Ba CaWO defining preferred compositions includes single phase compound Ba CaWO single phase Ba CaWO solid solution, or mixture of the Ba CaWO phase with binary phases as determined by the phase equilibrium relationships. The boundaries of the crosshatched area comprehend mixtures in the BaO- CaO-WO system containing 43-54 mole percent BaO, 20-30 mole percent CaO, and 21-27 mole percent WO Preferred compositions within the cross hatched area extend along the BaO--Ba CaWO join and its extension beyond the ternary compound and have a BaOzCaOzWO molar ratio extending from l.9:1:l to 2.l:l:l.

DESCRIPTION OF PREFERRED EMBODIMENT A high intensity sodium vapor discharge lamp in which the invention may be embodied is illustrated at l in FIG. 4 and comprises an outer vitreous envelope or jacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by a reentrant stem 4 having a press 5 through which extend stiff inlead wires 6,7 connected at their outer ends to the threaded shell 8 and center contact 9 of a conventional screw base.

The inner envelope or arc tube 1 1 is made of sintered high density polycrystalline alumina ceramic per US. Pat. No. 3,026,201 Coble, Transparent Alumina and Method of Preparation, or of other light-transmitting ceramic capable or withstanding the attack of sodium vapor at high temperatures. The ends of the tube are closed by thimble-like niobium metal end caps 12,13 hermetically sealed to the alumina by means of a glassy sealing composition which is shown exaggerated in thickness at 14 in FIG. 5.

Thermionic electrodes 15 are mounted in the ends of the arc tube. As best seen in FIG. 2, the electrode comprises an inner tungsten wire coil 16 which is wound over a tungsten shank 17 crimped or welded in the end of a niobium tube 18 welded through the end cap. The central turns in the inner coil 16 are spread apart and the outer tungsten wire coil 19 is screwed over the inner coil. The electron-emissive mix containing Ba CaWO may be applied to the electrode coils by painting or alternatively by dipping the coils in the suspension The material is retained primarily in the interstices between the turns of outer and inner coil and of inner coil and shank.

Lower tube 18 is pierced through at 21 and is used as an exhaust tube during manufacture of the lamp. After the gas filling and sodium mercury amalgam had been introduced into the arc tube, exhaust tube 18 is hermetically pinched off by a cold weld indicated at 22 and serves thereafter as a reservoir for condensed sodium mercury amalgam. Upper tube 18' has no opening into the arc tube and is used to contain a small quantity of yttrium metal (not shown) which serves as a getter; the end of the tube is closed by a pinch 23 which need not be hermetic. The illustrated lamp is limited to base down operation wherein the longer exhaust tube 18, which must be the coolest portion of the arc tube for the amalgam to condense therein, is located lowermost.

The are tube is supported within the outer envelope by means of a mount comprising a single rod 25 which extends the length of the envelope from inlead 7 at the stem end to a dimple 26 at the dome end to which it is anchored by a resilient clamp 27. End cap 13 of the arc tube is connected to the frame by band 29 while end cap 12 is connected to inlead 6 through band 30 and support rod 31. The interenvelope space is desirably evacuated in order to conserve heat; this is done prior to scaling off the outer jacket. A getter, suitably barium-aluminum alloy powder pressed into channeled rings 32, is flashed after scaling in order to assure a high vacuum.

In making the arc tube, the internal portions of the niobium metal end caps which engage the alumina tube are coated with a sealing composition comprising primarily aluminum oxide and calcium oxide and a minor proportion of magnesium oxide. The sealing composition is first applied to the end caps and then the end caps are assembled to the alumina tube and the parts placed in an electric vacuum furnace. The temperature is raised slightly above the melting point of the sealing composition which is upwards of 1400C. The electrodes may have previously been coated with the completely reacted Ba CaWO applied as a suspension in methanol, or alternatively, the unreacted materials may be applied as a suspension to the electrode and the reaction allowed to take place in the electric furnace simultaneously with sealing.

Table l below compares the performance of lamps such as described having the standard emission mix used heretofore with lamps having Ba CaWO in accordance with the invention. The standard mix consi'sted of barium thorate, BaThO ,to which is added 0.1 gram atom of thorium per mole, that is BaThO 0.1 Th.

Various compositions in the shaded area about the point Ba CawO in FIG. 3 have been tested and have given results superior to those from other emission materials.

High pressure sodium vapor lamps in particular give superior performance when using as emission material either pure Ba CaWO or materials with BaO:CaO:WO molar ratios of l.9:l:l to 2.l:l:l.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A high intensity electric discharge lamp comprising a light-transmissive envelope having electrodes sealed into its ends and containing an ionizable medium for carrying the discharge, said electrodes consisting of a refractory metal support structure and electron emissive material applied thereto consisting of composites in the BaO-CaO-WO system containing 43-54 mole percent BaO, 20-30 mole percent CaO, and 21-27 mole percent W0 2. A lamp as in claim 1 wherein the electron emissive material consists of BaO1CaOzWO in molar ratios extendingfrom l.9:l:1 to 2.1:]:1.

3. A lamp as in claim 1 wherein the electron emissive material consists of a Ba CaWO solid solution phase.

4. A lamp as in claim 1 wherein the electron emissive material consists of essentially Ba CaWO 5. A high intensity sodium vapor discharge lamp comprising a slender tubular elongated ceramic envelope, a pair of electrodes sealed into the ends of said envelope, a filling of sodium, mercury and an inert gas within said envelope, each electrode comprising a tungsten wire coil to which is applied an electron emissive material filling the interstices between turns of the coil, said electron emissive material consisting of a refractory metal support structure and electron emissive material applied thereto consisting of composites in the BaO-CaO-WO system containing 43-54 mole percent BaO, 20-30 mole percent CaO, and 21-27 mole percent W0 l 6. A lamp as in claim 5 wherein the electron em1ss1ve material consists of BaOzCaOzWO in molar ratios extendingfrom l.9:1:l to 2.12121.

7. A lamp as in claim 5 wherein the electron emissive material consists of a Ba CaWO solid solution phase.

8. A lamp as in claim 5 wherein the electron emissive material consists of essentially Ba CaWO 

2. A lamp as in claim 1 wherein the electron emissive material consists of BaO:CaO:WO3 in molar ratios extending from 1.9:1:1 to 2.1:1:1.
 3. A lamp as in claim 1 wherein the electron emissive material consists of a Ba2CaWO6 solid solution phase.
 4. A lamp as in claim 1 wherein the electron emissive material consists of essentially Ba2CaWO6.
 5. A high intensity sodium vapor discharge lamp comprising a slender tubular elongated ceramic envelope, a pair of electrodes sealed into the ends of said envelope, a filling of sodium, mercury and an inert gas within said envelope, each electrode comprising a tungsten wire coil to which is applied an electron emissive material filling the interstices between turns of the coil, said electron emissive material consisting of a refractory metal support structure and electron emissive material applied thereto consisting of composites in the BaO-CaO-WO3 system containing 43-54 mole percent BaO, 20-30 mole percent CaO, and 21-27 mole percent WO3.
 6. A lamp as in claim 5 wherein the electron emissive material consists of BaO:CaO:WO3 in molar ratios extending from 1.9:1:1 to 2.1:1:1.
 7. A lamp as in claim 5 wherein the electron emissive material consists of a Ba2CaWO6 solid solution phase.
 8. A lamp as in claim 5 wherein the electron emissive material consists of essentially Ba2CaWO6. 